Magic is easier, IMO, to base a traditional storyline around as the 'rules' of magic are squiffy and ill-defined (and can be mangled to suit the storyteller's convenience).
Higher levels of technology are harder and harder to handwave away, and many, if not most, traditional adventure storylines, would fall apart if one of the protagonists had a cellphone or whatever, let alone supertech stuff like transporters (which end up having to oh-so-conveniently fail due to an 'ion storm' or whatever whenever they are needed...). "Oh, we're trapped in an exotic hostile location!" "Get out your cellphone and call for help." "Oh, there's a killer in the house!" "Call 911 and hide until the cops show up." "We have to figure out this puzzle, quick everyone rack your brains..." "Right, I just Googled it. It's the Voynich manuscript." "I can't get the door open, we're trapped!" "According to the online help menu, there's a manual release under the jamb, in case of power failures. Also, I called a locksmith."
Fantasy settings also have a wider range of unique characters in the 'party.' If one wants to play a wizard and another wants to play an archer and another wants to play a sneaky thief, fantasy can keep them all in the same room. In a more futuristic game, everyone ends up being specialists of the same 'class,' whether they are starship troopers or mechwarriors or viper pilots or a star trek away team. There ends up being less flexibility, and there are increased chances that one or more of the characters have abilities that mostly function when they are solo (whether a psychic in dream space, or a mechwarrior in his mech, or a rigger in his hovertank, or a cyberdecker in the virtual realm), leaving the rest of the party behind. It makes it harder to run a single story for four or five people, if half of them have abilities that work best when they are nowhere near each other, entering specialized environments (like cyberspace) or vehicles (like mecha).
Even when I play sci-fi settings (which I do love, especially Trinity), I tend to end up playing 'magical' characters like psychics or alien shapeshifters or whatever, rather than 'dude with gun #1428.' Dudes with guns bore me, and that's, unfortunately, what every single viper pilot or away team member turns into in the end, just another dude with a gun.